A fire damper of the type referred to typically includes a frame which may be circular or rectangular, and a pair of blades pivotally coupled the frame about parallel axes that extend across the center of the frame. Both blades can be moved between closed positions in which they extend generally in the plane of the frame and open positions in which they pivot towards one another at the same side of the frame. A damper of this configuration is often referred to as a "butterfly" damper.
A damper of this type is usually mounted in an air duct and is designed to interfit with sections of the duct. The blades are spring-biassed towards their closed positions but are normally held open by a mechanism mounted on a bar that extends across the frame parallel to the blade pivot axes. The mechanism is designed to release the blades in the event of a fire and permit them to close under the effect of their spring biassing.